UPSTREAM: arm64: map [_text, _stext) virtual address range non-executable+read-only

Since the referenced fixes commit, the kernel's .text section is only
mapped starting from _stext; the region [_text, _stext) is omitted. As a
result, other vmalloc/vmap allocations may use the virtual addresses
nominally in the range [_text, _stext). This address reuse confuses
multiple things:

1. crash_prepare_elf64_headers() sets up a segment in /proc/vmcore
   mapping the entire range [_text, _end) to
   [__pa_symbol(_text), __pa_symbol(_end)). Reading an address in
   [_text, _stext) from /proc/vmcore therefore gives the incorrect
   result.
2. Tools doing symbolization (either by reading /proc/kallsyms or based
   on the vmlinux ELF file) will incorrectly identify vmalloc/vmap
   allocations in [_text, _stext) as kernel symbols.

In practice, both of these issues affect the drgn debugger.
Specifically, there were cases where the vmap IRQ stacks for some CPUs
were allocated in [_text, _stext). As a result, drgn could not get the
stack trace for a crash in an IRQ handler because the core dump
contained invalid data for the IRQ stack address. The stack addresses
were also symbolized as being in the _text symbol.

Fix this by bringing back the mapping of [_text, _stext), but now make
it non-executable and read-only. This prevents other allocations from
using it while still achieving the original goal of not mapping
unpredictable data as executable. Other than the changed protection,
this is effectively a revert of the fixes commit.

Bug: 448550230
(cherry picked from commit 5973a62e)
Fixes: e2a073dd ("arm64: omit [_text, _stext) from permanent kernel mapping")
Cc: stable@vger.kernel.org
Change-Id: Ibc5068e79f47f303dfc3bde6258a2f6844d26a15
Signed-off-by: Omar Sandoval <osandov@fb.com>